When printing a colored image onto a receptor material such as a plastic credit card, three primary colors are typically used to create the composite color image. This is typically accomplished in three separate printing passes using a print head to print each of the three primary colors successively from a ribbon onto the credit card. A printing pass is commonly accomplished by moving the credit card across a fixed print head. The credit card is typically moved either by the use of a set of rollers acting upon the surface of the card or by carrying the card along a carriage yoke.
There exist several known devices for transporting a workpiece such as a credit card. As above, the card may be urged forward through the use of various rubber roller sets, or less commonly, through the use of hardened pins. The roller sets may be used in conjunction with upper and lower guide rails to maintain card position stability. The carriage yoke is typically provided with leading and trailing fingers for retaining the card, and may be driven via a steel cable and roller set. Other known devices for transporting the card include a tabbed timing belt with a plastic finger for driving the card into and out of the device, and urethane belts situated along the front and back sides of the card which are also used in conjunction with roller sets.
One problem encountered with some of these methods of transporting a card in association with a printing operation is the difficulty in being able to accurately line up the printing of each color on the card during each pass of the card before the print head so that the image has the desired clarity and resolution, particularly when printing tonal color images at the high rates attainable by typical card printers. Another problem is that full card printing is restricted either to a single pass of the print head or that the print head has only partial access to the desired face of the card. In a printer apparatus in which the printing is accomplished using a thermal print head, it is necessary that the full card surface be clear of all obstructions to enable printing on any portion of the card surface; however, it is equally necessary that some form of card holding device be used to retain the card in proper alignment with the print head.
Alternative card holding devices include vacuum technology and mechanical clamping devices. Vacuum technology requires numerous components such as a vacuum pump, control valve, and an expanded system controller board. Vacuum pumps produce an extra heat load and audible noise, and present additional problems in accommodating the components within the physical confines of existing card handling systems. To mechanically retain the card without obstructing the face of the card would require means for holding the card edges, which would cause the card to bow away from the retainer surface. Furthermore, to truly secure the card by these means would necessitate wrapping the mechanical retention means onto the printed side of the card, thereby interfering with the printing process. Additionally, mechanical means would need to be provided for removing the card from the card holding devices
Thermal print heads such as thin film, thick film, and edge print heads consist of a line of resistive heating elements protected by a wear resistant layer. The resistive elements generate heat when a voltage is applied. Thermal print heads generally consist of one or more 1.times.N bit shift registers with an output stage capable of providing the power necessary to energize a resistor, where N is either the maximum width or height of the image in pixels. Data is clocked into the print head one bit at a time. The number of shift registers used depends on the particular thermal print head used.
Digital continuous tone pixels use multi-bit values, which requires that a print head be loaded many times to print a single row of an image. For instance, if six bits are used to store intensity levels for the image, the print head must be loaded 2.sup.6 or 64 times.
Most applications control the print head directly by a microprocessor. The microprocessor provides both the data stream, clock, and other control signals required by the print head. Because the microprocessor has other duties such as managing print data and controlling other printer functions, the speed of print head operations is usually below its maximum and can exhibit irregularities depending on how busy the microprocessor is. This slow down is even more noticeable with continuous tone printing due to the large amount of data that must be sent to the print head.
Therefore, there is a need to provide a way to increase the print speed of a print head to near its maximum amount.
The present invention solves these and other problems associated with a card transport device and print head controller for use in a printer.